Intraneuronal amyloid-p peptide (AP) accumulation is an early and toxic event in the pathogenesis of Alzheimer's disease (AD). Understanding cellular mechanisms that accelerate or inhibit intraneuronal A[unreadable] accumulation may provide novel therapeutic strategies for AD. A[unreadable] can accumulate inside neurons via receptor-mediated uptake. It can also accumulate via de novo processing of amyloid precursor protein (APR) to A[unreadable] in the endocytic pathway. Our recent studies have shown that apolipoprotein E (apoE) receptors, members of the low-density lipoprotein receptor (LDLR) family, modulate A[unreadable] uptake as well as APR endocytic trafficking and processing to A[unreadable]. In particular, we have demonstrated that LRP overexpression in the brain increases cell-associated A[unreadable]. A[unreadable] can bind to apoE receptors either directly or indirectly via A[unreadable] chaperones such as apoE. This proposal will focus on two apoE receptors, the LDLR-related protein (LRP) and LRP1B. These homologous receptors are both highly expressed in neurons and bind multiple ligands including A[unreadable], apoE, and APP. However, evidence from our lab suggests that LRP and LRP1B play opposing roles in ligand endocytosis. While LRP mediates rapid endocytosis, LRP1B endocytoses very slowly and as a consequence, retains ligands at the cell surface. Our overall hypothesis is that LRP facilitates A[unreadable] uptake, p production, and intraneuronal A[unreadable] accumulation, and that LRP1B blocks these effects, thus inhibiting A[unreadable] toxicity and pathogenesis of Alzheimer's disease. We have designed both in vivo and in vitro approaches to test our hypothesis. In Aim 1, we plan to determine the roles of LRP and LRP1B in intraneuronal accumulation in animal models. Because conventional LRP knockout is early embryonic lethal, our lab has generated conditional LRP forebrain-specific knockout mice. Together with the LRP1B knockout mice, we plan to test the roles of LRP and LRP1B in intraneuronal A[unreadable] accumulation after brain A[unreadable] infusion or after breeding with PDAPP amyloid model mice. In Aim 2, we will define the opposing roles of LRP and LRP1B in apoE-dependent and apoE-independent A[unreadable] uptake in primary neurons. Impacts of altered LRP and/or LRP1B expression in neurons on A[unreadable] uptake and intraneuronal accumulation will be assessed in the absence or presence of apoE. In Aim 3, we plan to dissect the mechanisms underlying the opposing roles of LRP and LRP1B in APP endocytic trafficking and A[unreadable] production. Our proposed studies will take advantage of our experience in studying the cell biology of receptor-mediated endocytosis, APP endocytic trafficking, and A[unreadable] metabolism using both in vitro and in vivo approaches. Because recent studies have established the causal role of intraneuronal A[unreadable] in cognitive deficits prior to A[unreadable] plaques, an understanding of the pathways leading to or protecting against intraneuronal AB accumulation may lead to specific targets for AD therapy.